Fireballs formed in heavy-ion collision rapidly expand and by the time the system is thermalized the temperatures do likely not exceed a few hundred  M\(e\) V.
It is understood that at such temperatures the plasma is not close to an ideal plasma.
To investigate the impact of a (strong) coupling of the plasma on the emissivity, higher order effects have been studied (see e.g. for a concise overview [1]}).
A step forward in deriving the vector correlator in a partonic medium, beyond the perturbative approach, has been achieved by introducing so-called hard-thermal-loops (HTL) [2]}, [3]}.
This concept overcomes the detailed book-keeping of Feynman diagrams otherwise needed to avoid singularities occurring as the invariant mass approaches zero.
The vector correlator can also be evaluated on the lattice.
For that, the spectral properties of the correlator have to be extracted from the correlator calculated in Euclidean space-time.
The spectral function is then obtained from a Fourier transformation of the Euclidean correlator.
Technically, this is achieved by employing Maximum Entropy Methods (MEM) as the correlator is only known on the intersections of the space time grid.
Such calculations have been carried out e.g. by the Hot QCD collaboration within the quenched approximation, i.e. using static quarks only [4]}.
The resulting dilepton rates as a function of the dimensionless ratio of dilepton energy to temperature for virtual photon momentum \(p=0\)  is shown in the left panel of Fig. REF .
The vector correlator has recently been calculated also with dynamical quarks for 2+1 flavors using anisotropic lattices [5]}.
This calculation addresses a range of temperatures crossing the pseudo-critical temperature.
For temperatures below \(T_{\mathrm {pc}}\) , a clear resonance peak is obtained at around 800  M\(e\) V/\(c^{2}\) for the vector correlator using \(m_q = m_{u,d}\)  (see Fig. REF  (right)).
Once the temperature of the radiating source drops below the deconfinement temperature (cf. Sec. REF ), the spectral distribution of the thermal radiation is modulated due to the presence of hadronic resonances as depicted in the right panel of Fig. REF .
